There is, but there's no easy way. I was working on a project that uses a fog of war and the way that I did it was I created a 'fog' TileLayer that went over top of the ground tiles game. Any tile that is visible by the current player gets set to a transparent tile, whereas any tile that is not visible is set to a darker tile. You can use the TileLayer's setTile function to change the tile when you need to.
You can take a look at my game (it's very incomplete). Load it up in two windows so it 'matches' you with an opponent so the game will begin.
Here's the GitHub repository: github.com - waroflords
You should find how it works mainly in game.js at around line 85 and 191.
Keep in mind, this is tile-based fog of war, so the fog will always be tile shaped (at this point).